We have constructed a microcalorimeter which enables us to determine heats of hydrogenation of very small samples of unsaturated substances. We have obtained accurate heats of hydrogenation of samples smaller than eight microliters. This represents a vast increase in sensitivity over previous hydrogen calorimeters. Standard errors of several series of determinations in our laboratory were usually between 0.5 and 1.0%. Greatly reduced sample size enables us to obtain hydrogenation data on molecules which are difficult to obtain, difficult to purify or both. Since its completion we have used the calorimeter to determine the heats of hydrogenation of about 100 compounds including strained molecules, geometric isomers, fatty acid methyl esters, simple triglycerides and carcinogens. The results were interpreted in terms of the geometric, conformational and quantum mechanical properties of the molecules. Although our basic aim is to concentrate on molecules of biochemical and biomedical importance,we have devoted a considerable amount of work to determining heats of hydrogenation of smaller molecules so as to verify the method by amassing a large body of data which can be cross-checked by comparison with older hydrogenation and combustion data. A biproduct of this research was the development of hydrogen enthalpimetry which is described in several publications. We propose to extend the method to many more unsaturated species, such as steroids, lipids, some vitamins and carcinogens.